Solar heat gain of insulation (not windows)
It’s great that the industry publishes the R value of different insulating materials (fiberglass, Rockwool, wood fiber, different foams, hemp, sheep wool, cellulose, and on and on), but I’m realizing that R value is a pretty limited data point that doesn’t tell enough of the story, especially when it comes to sunny days versus cloudy days.
During the last couple of weeks, we’ve had sunny days at about 80 degrees F, and cloudy days with the same air temperature. The difference in how the sun versus clouds affected my house was enormous. I have a decent amount of insulation in the roof (R60-ish; I have to say “ish” because it varies; I’m in the middle of an energy retrofit, and some parts are easier to insulate than others), and with the same amount of insulation across all of these days, the cloudy 80 degree days were comfortable, with the air conditioning running very occasionally, but not much. On the sunny days, the air conditioning ran all day, non-stop.
We could say that I should add insulation to the walls, which currently have none, and that is definitely part of my plan for the future, but honestly, the uninsulated walls performed well enough on the cloudy days. The only difference that really mattered was the sun.
In building science there is a whole lot of talk of air tightness, R value, and there are even discussions of the effect of wind outside or air flow within the insulation material… and there is plenty of talk of solar heat gain for windows, but there doesn’t seem to be much talk of solar heat gain for insulation, aside from windows. I’d love to see a solar heat gain coefficient (SHGC) data point — I suppose it would have to be SHGC per inch) for Rockwool, wood fiber and all the rest.
A sort of related idea is the concept of thermal mass, where it helps to have high thermal mass outside of the enclosure on a hot day, to absorb the heat during the day, then release it at night, when it is cooler, though if the difference in temperature between day and night is small, all the heat stored in that thermal mass will eventually be passed to the inside of the enclosure.
And I know there are radiant heat barrier products, which do get discussed sometimes, though often dismissively, because they have essentially no R value, which is true, but if I had a huge shade umbrella (or tree or radiant barrier product) over my house on the hot sunny days, my AC would not have run hardly at all, assuming that the shade is as effective as the clouds. I suppose that means that the shade would have to encompass my whole house, to prevent the sun from hitting the walls and windows, and to prevent reflected radiant heat from bouncing off the ground or neighboring houses.
So it gets complicated, I know, but still: I want a SHGC measurement for insulation, because I can now confidently say that on sunny days, solar heat gain (absorbed radiant heat, or perhaps (?) more accurately the “emissivity” of the insulation) matters a whole lot more than R value. The sun overpowers the insulation far more easily than high outdoor temperatures do.
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On the flip side of everything i said in the original post, during sunny winter days, the best "insulation" -- at least while the sun is shining directly on the house -- is probably just glass, with an R value of about 1, along with some thermal mass inside to absorb the heat. I realize that I'm just stating an old passive heating principle here, and the problem in cold seasons is the cloudy days and the sunless nights.
But even with high R value insulation, I'm realizing that cold sunny days still transfer a lot of heat into the house through the insulation. There would still be a lot of solar heat gain despite the insulation, just because of how powerful the sun is.
The sunlight gets converted to heat when it hits the side or roof of the house. How much is absorbed by the house versus reflected is determined by the surface, not the insulation behind it. Once that surface absorbs the heat it is dispersed in all directions, how much goes toward the inside of the house instead of the outside air is determined by the amount of insulation in the wall.
So if we were to run a controlled experiment comparing radiant heat bearing down directly on insulation versus heated air vs a heated object applied directly to the insulation (which is how they measure R value, as I understand it), you don’t think there would be any measurable difference, assuming the energy of all the heat sources is equivalent? You think it all comes down to just R value, and there is no radiant absorption or radiant penetration effect that can be measured separately?
There's no reason to expect that insulation would have radiation penetration. Radiation transfers heat through light, if light is blocked there is no radiation.
Let's say you have a wall, with an outer surface, an insulation layer, and an inner surface. The outer surface is at a certain temperature, the inner surface is at a certain temperature. I would see no reason for the heat transfer through the insulation to be different if the outer surface is being warmed by radiation.
For example, image the outer surface is at 95F and the inner is at 75F. I would expect the heat transfer through the wall to be the same if the outdoor temperature is 95F and the wall is in the shade as if the outdoor temperature is 50F and the wall is in direct sunlight and warmed up to 95F.
Ok, so doing a little more internet sleuthing, it seems that the components of a wall, including insulation, will block almost all direct infrared light/radiation, as you said, but there are some interesting exceptions of objects that are transparent to the human eye that block infrared, and some objects that are opaque to the human eye but that don’t block infrared. That said, none of those materials are relevant to the construction of walls or ceilings, as far as I can tell.
And just for clarification, even though infrared radiation is indeed a form of light, it is invisible. Any hot object gives off invisible infrared light, even in the dark. A black tarp suspended over a roof would block the sun’s infrared radiation, but absorb it, turn it into heat, and emit it out again, so the roof below the tarp would still receive radiant heat, but from the tarp, not the sun.
I’m just talking through this for my own benefit at this point, because it’s interesting to think through how it all works.
Good quality windows will have a coating to block IR and allow visible light through.
In your tarp example, yes the roof would receive some radiation from the tarp. But since the tarp is much closer to the roof temperature than to the sun temperature the tarp would radiate much less to the roof than it receives from the sun. Most of the heat it receives would be lost to the surrounding air.
Here's another counter-intuitive aspect: if you have a window that allows in sunlight, the sunlight will hit an object in the room and release heat. What you might not expect is that it doesn't matter what that object is made of or where it's positioned. Unless the actual light is reflected back out through the window, the heat is going to stay in the room, even if it bounces around a bit first. Painting the walls black doesn't increase the solar gain, and putting a shade on the inside of the glass doesn't reduce it.
"putting a shade on the inside of the glass doesn't reduce it"
It would be interesting to take a thermal camera and shoot some identical windows side by side from the outside- one with an interior shade, one without. The glass coating might make that experimetn difficult.
I suspect that the one with the shade would be hotter, because that solar gain is trapped between the window and the shade, thereby increasing exit raditation.
Technically it is all "solar gain" once in enters the room, so you are correct, but I'm guessing that there is some (even if mostly neglible) cooling effect by having shades (due to increased infrared radiation exiting the building).
There's probably some reflection back through the glass. And some increased heat loss because the area next to the glass is warmer. But the point is that shading a window on the inside isn't an effective way of preventing insolation. Although it does make occupants more comfortable because the sun isn't right on them, they're not the object absorbing and radiating that heat.
The other big point is that what's in the room doesn't matter. I read all the time about people who put stone or concrete or black paint in their interior to try and capture more solar energy. Everything coming through the window is getting captured, you don't have to do that.